It is often necessary to protect high temperature articles with an Environmental Protection Coating (EPC). EPC's have insulative and chemical protection capabilities to protect surfaces from the harsh environment.
EPC's are typically applied to hot sections of gas turbines or jet engines, such as combustor cans, nozzle guide vanes, and turbine blades. One of the functions of these coatings is to increase engine efficiency by elevating the operational temperature or reducing the need for cooling air. The use of thermal barrier coatings in large turbines for land-based power generation is critically necessary for an acceptable operating lifetime. EPC's and cooling mechanisms are often used in turbine systems to protect the metallic parts that comprise turbines. The EPC enables extension of component life and improved reliability by lowering the operating metal temperature, thereby also lowering loss of strength and oxidation. As such, costs are reduced by eliminating elaborate cooling schemes required for metals in high temperature applications. The increased maximum gas temperature permitted by the EPC insulating and chemical protective capabilities provides significant performance improvement and thus large cost savings by increasing the turbine inlet temperatures. Efficiency improvements are thus limited by the capabilities of the EPC applied to turbine parts, such as the blades. Higher temperature EPC's may also enable similar efficiency improvements in small radius hypersonic aircraft leading edges, higher thermal efficiency engine components and exhaust washed surfaces.
The life and performance of high speed aircraft may also be improved by the application of EPC's. As with the turbine blades, the EPC's provide protection from heat and oxidation within the limits of the EPC's. The application of EPC's on forward facing surfaces is especially important for hypersonic aircraft due to the pronounced aero-thermal heating that occurs at those speeds. The operational speed of the wing and thus the aircraft is thus also limited by the capabilities of the EPC applied to the leading edges.
The current EPC's used at temperatures approaching 3000 degrees F. are typically silica-sealed ceramics. Hypersonic edge coatings are typically silica while turbine blades are typically Zirconia based. The ceramic coating may employ refractory oxides, nitrides, borides or carbides, to provide the thermal barrier. Refractory materials may include compounds of Al, Si, Zr, Hf, and Ta, among others. Silicon-based coatings, which oxidize to silica glasses, have capabilities of between 2400 to 3000 degrees F., depending on the lifetime required, can remain effective for up to thousands of hours or for short term single use, respectively. At ultra high temperatures, i.e. above 3000 degrees F., the current EPC's degrade rapidly.
Attempts to develop higher temperature EPC's, however, have been met with various material deficiencies as shown below:
1. Sealant qualities: In order to ensure initial and in-use requirements to impede the ingress of oxygen to the substrate, the coating must be able to form appropriate sealing glasses (oxides) to seal off any cracks, pin-holes or porosity that may develop from chemical activities or physical stresses. Current high temperature EPC's do not provide adequate sealing from oxygen at ambient, intermediate and ultra high temperatures.
2. Compliance: High thermal strains are typically experienced in weak and high modulus EPC ceramics. This strain creates stresses greater than constituent failure strength for high CTE or temperature differential, causing spalling and cracking
3. Volatility: Preferred sealing materials have high vapor pressures at the temperatures of interest; some, like silica, enter a regime of active oxidation and rapidly degrade. Very high internal vapor pressure can push aside viscous sealants creating pin-holes.
4. Mis-matched Coefficient of Thermal Expansion (CTE): Protective coatings expand at a different rate than the substrate when heated and generate very high stresses leading to cracks, pin-holes and spalling.
5. Chemical incompatibility: EPC constituents capable of producing environmentally stable compounds may be reactive with the substrates.
6. Adhesion: Existing EPCs may not adhere well to the substrate.
Attractive compounds exist that may be useful in developing improved EPC's. Inter-metallic MAX phase compounds are ternary carbides and nitrides with the general formula Mn+1AXn (MAX) with n=1-3. M is an early transition metal, A is an A-group element (predominantly IIIA and IVA in the periodic table) and X can be carbon and/or nitrogen. These compounds behave like metals regarding their machinability and their thermal and electrical conductivities but behave like ceramics in terms of stiffness, oxidation resistance, thermal stabilities and high melting points. However, even these attractive materials do not have the breath of properties required for ultra high temperature environments as they are poly-crystalline, develop high thermal stresses, tend towards cracking and oxidation at grain boundaries, providing pathways for oxygen to reach the substrate surface. Use of such materials would require an integrated material engineering solution combining material characteristics in advantageous micro-structures by further processing.
There is thus a continuing and pressing need for improved EPCs so as to advance the efficiency and life of articles subjected to ultra high temperatures.